Liquid dairy products, such as milk, may be thermally processed to increase their stability. Unfortunately, thermally treating milk often results in color changes and/or gelation during processing or extended storage. For example, lactose in milk heated to high temperatures tends to interact with proteins and results in an unsightly brown color. This undesired condition is often referred to as “browning” or a “browning reaction.” Gelation, on the other hand, is not completely understood, but the literature suggests that gels may form, under certain conditions, as a three-dimensional protein matrix formed by the whey proteins. See, e.g., Datta et al., “Age Gelation of UHT Milk—A Review,” Trans. IChemE, Vol. 79, Part C, 197-210 (2001). Both gelation and browning are undesirable in milk since they impart objectionable organoleptic properties. Although a limited amount of browning can be accepted, it is preferred that no gelation or protein aggregation is visible.
The concentration of milk is often desired because it allows for smaller quantities to be stored and transported, thereby resulting in decreased storage and shipping costs, and may allow for the packaging and use of milk in more efficient ways. However, the production of an organoleptically-pleasing, highly concentrated milk can be difficult, because the concentration of milk generates even more pronounced problems with gelation and browning. For instance, milk that has been concentrated at least three fold (3×) has an even greater tendency to undergo protein gelation and browning during its thermal processing. Additionally, such concentrated milk also has a greater tendency to separate and form gels over time as the product ages, thereby limiting the usable shelf life of the product. Concentrated milk, as a result, is generally limited to concentrations below about 25 percent total solids, protein levels below about 7 percent, and a shelf life of less than 6 months.
Numerous studies have been reported on gelation and browning of milk and concentrated milk and many factors affecting gelation in milk have been identified. Examples of such factors include calcium (chelation and/or removal), mode and severity of thermal treatment, proteolysis, milk production factors, microbiological quality of raw milk, storage temperature and time, additives, fat content, pH, and the polymerization of casein. See, e.g., Udabage et al., “Effects of Mineral Salts and Calcium Chelating Agents on the Gelation of Renneted Skim Milk,” 84:1569-1575 (2001); Cano-Ruiz et al., “Changes in Physicochemical Properties of Retort-Sterilized Dairy Beverages During Storage,” J. Dairy Sci. 81:2116-2123 (1998); El-Din et al., “Polymerization of Casein on Heating Milk,” Int. Dairy J. 3:581-588 (1993); McMahon et al., “Effects of Phosphate and Citrate on the Gelation Properties of Casein Micelles in Renneted Ultra-high Temperature (UHT) Sterilized Concentrated Milk,” Food Structure, Vol. 10, 27-36 (1991); Harwalkar et al., “Effect of Added Phosphates and Storage on Changes in Ultra-High Temperature Show Time Sterilized Concentrated Skim Milk. 1. Viscosity, Gelation, Alcohol Stability, Chemical and Electrophoretic Analysis of Proteins,” Neth. Milk Dairy J. 32:94-111 (1978).
The production of concentrated milk, also known as evaporated milk, is known in the art and may be produced from whole milk, partly skimmed milk, or skim milk. Unfortunately, as noted above, the concentration and shelf life of typical concentrated milk may be limited due to gelation and browning problems. Typically, as noted above, concentrated milk products are limited to less than 25 percent total solids, less than 7 percent protein, and have shelf lives of less than 12 months, and often significantly less, due to age gelation.
A typical method of producing concentrated milk involves multiple heating steps in combination with the concentration of the milk. For example, one general method used to produce concentrated milk involves first standardizing the milk to a desired ratio of solids to fat and then forewarming the milk to reduce the risk of the milk casein from coagulating during later sterilization. Forewarming also decreases the risk of coagulation taking place during storage prior to sterilization and may further decrease the initial microbial load. The forewarmed milk is then concentrated by evaporation, ultrafiltration, or other appropriate methods to the desired concentration. The milk may be homogenized, cooled, restandardized, and packaged. In addition, a stabilizer salt may be added to help reduce the risk of coagulation of the milk that may occur at high temperatures or during storage. Either before or after packaging, the product is sterilized. Sterilization usually involves either relatively low temperatures for relatively long periods of time (e.g., about 90 to about 120° C. for about 5 to about 30 minutes) or relatively high temperatures for relatively short periods of time (e.g., about 135° C. or higher for a few seconds).
The degree of sterilization or the sterilization value (Fo) is based on the time that the dairy product is subjected to specific temperatures and is a culmination of all thermal treatments that the product encounters during processing. Consequently, a desired sterilization value may be achieved through a variety of processing conditions. Typically, concentrated milk is sterilized to a Fo of at least 5 and preferably to a much higher level (e.g., 15 or higher). Unfortunately, as discussed above, high temperatures or long exposures to elevated temperatures, as are generally necessary in conventional sterilization methods to achieve the desired sterilization values, also adversely affect the long term stability of concentrated milk, especially concentrated milk with greater than about 7 percent protein, by inducing gelation or browning.
The sterilization value for a sterilization process can be measured using graphical integration of time-temperature data during the food's slowest heating point rate curve for the thermal process. This graphical integration obtains the total lethality provided to the product. To calculate the processing time required to achieve a desired Fo using the graphical method, a heat penetration curve (i.e., a graphical plot of temperature versus time) at the slowest heating location of the food is required. The heating plots are then subdivided into small time increments and the arithmetic mean temperature for each time increment is calculated and used to determine lethality (L) for each mean temperature using the formula:L=10(T-121)/z 
Where:                T=arithmetic mean temperature for a small time increment in ° C.;        z=standardized value for the particular microorganism; and        L=lethality of a particular micro-organism at temperature T.        
Next, the lethality value calculated above for each small time increment is multiplied by the time increment and then summed to obtain the sterilization value (Fo) using the formula:Fo=(tT1)(L1)+(tT2)(L2)+(tT3)(L3)+ . . .
Where:                tT1, tT2, . . . =Time increment at temperature T1, T2, . . . ;        L1, L2, . . . =Lethality value for time increment 1, time increment 2, . . . ; and        Fo=Sterilization value at 121° C. of a microorganism.        
Consequently, once a penetration curve is generated, the sterilization value Fo for the process can be computed by converting the length of process time at any temperature to an equivalent process time at a reference temperature of 121° C. (250° F.). Jay, 1998, “High Temperature Food Preservation and Characteristics of Thermophilic Microorganisms,” in Modern Food Microbiology (D. R. Heldman, ed.), ch. 16, New York, Aspen Publishers.
Various approaches for the production of concentrated milk have been documented. For example, Wilcox, U.S. Pat. No. 2,860,057, discloses a method to produce a concentrated milk using forewarming, pasteurizing, and high-temperature, short-term sterilization after concentration. Wilcox teaches the concentration of milk to approximately 26 percent solids using forewarming at about 115° C. (240° F.) for about 2 minutes prior to concentration, preheating at 93° C. (200° F.) for about 5 minutes after concentration, and sterilization at about 127 to 132° C. (261 to 270° F.) for 1 to 3 minutes.
Blake, U.S. Pat. No. 4,282,262, is directed to a method to produce dairy based mixes for frozen desserts. Blake discloses a milk-blend fraction comprising a specially prepared concentrated blend of milk, sugar, stabilizer salts, and casein-reactive gums. Blake teaches the concentration of a milk having between about 1 to 9 percent fat and added stabilizer salts to about 25 to 36 percent total solids, after which the various other components are blended therein. Initially, forewarming is continued until the milk has a standard whey protein nitrogen test ranging from 4.5 to 5.5. The concentrated milk blend is then sterilized by heating at 104 to 148° C. (220 to 300° F.) for 1 to 8 seconds.
Reaves et al., U.S. Patent Publication 20030054079 (Mar. 20, 2003), discloses a method of producing an ultra-high temperature milk concentrate having 30 to 45 percent nonfat milk solids. Reaves et al. teach the preheating of milk for 10 minutes at 65° C. (150° F.) to produce a preheated, milk starting product, which is then pasteurized at 82° C. (180° F.) for 16 to 22 seconds and evaporated under elevated pasteurizing temperatures (i.e., 10 minutes at 62° C. (145° F.) under vacuum) to produce an intermediate, condensed liquid milk. A cream and stabilizer, such as sodium hexametaphosphate or carrageenan, are added to the intermediate milk, which is then ultrapasteurized in two stages wherein the first stage is at 82° C. (180° F.) for 30 to 36 seconds and second stage is at 143° C. (290° F.) for 4 seconds. Shelf lives of 30 days to 6 months are reported for the resulting milk concentrate.
As indicated, concentrated milks require thermal processing to sterilize. The use of such elevated temperatures and increased exposure to such temperatures are factors that may contribute to undesirable properties in the milk, such as gelation and browning. Unfortunately, higher concentrations, such as protein levels greater than about 7 percent that are desired for efficiency and logistical standpoints, often make these undesirable conditions even more pronounced and difficult to avoid. Consequently, there is a desire for improved concentrated milks (generally 3× or higher and containing more than 7 percent protein) that are non-gelling and non-browning for greater than about 6 months storage at ambient conditions. There is also a desire for improved methods to produce such concentrated milks using a thermal treatment sufficient to sterilize and at the same time prevent gel formation and minimize browning. The present invention provides such compositions and methods.